This invention relates to protection arrangements for electrical power supply systems of the type which operate by detecting changes in impedance levels.
In electrical power transmission and distribution systems, it is customary for each end of each section of the system, such as a transformer, a power line and the like, to be provided with a protective device which disconnects its associated section if a fault occurs therein. It is, of course, important for disconnection to take place only if the fault occurs within the section.
Protection devices of this type are based on measuring, from a knowledge of the variation of the voltages and currents in each phase at a measuring point, the impedance presented to a protective relay. Under fault conditions, the impedance measured generally differs from that under no-fault conditions and it is possible to use this fact to trip one or more poles of a circuit breaker if a system fault develops.
Impedance is only meaningful when related to steady-state sinsusoidal conditions. In the interval immediately following a fault (typically one cycle of power frequency after fault inception), the waveforms derived from the section containing the fault contain a variety of transients which vary according to the power system operating conditions and configuration.
One consequence of the use of increasingly high voltages in transmission systems has been to increase the fault currents which can be produced and therefore reduce the maximum accpetable fault clearance time. It is therefore desirable to provide a protection arrangement which can operate using information derived during one cycle after fault inception. A system which operates within one cycle may be termed an ulta high speed (u.h.s) protection system. To operate within such time the measurement process should be completed within approximately 4.7 ms after a fault.
In UK Pat. No. 2016833, which formed the basis for U.S. Pat. No. 4,261,038 issued Apr. 7, 1981, to the same inventors as the present invention, there is described an apparatus for detecting the occurence of a fault in a section of an electrical power distribution system comprising means for measuring the instantaneous values of the voltage and current at a predetermined location, means for calculating an approximation to the Fourier transform of a window of the actual voltage waveform over a predetermined interval of time and a predetermined frequency, means for calculating an approximation to the Fourier transform of a window of the actual current waveform over said predetermined time interval and frequency, means for utilising the results of the calculation relating to the actual current waveform for calculating an approximation to the Fourier transform of the modified current which would flow if the voltage on each side of the time interval was zero, and means for calculating the actual impedance of the section from said Fourier transforms of the actual voltage and the modified current.